[sdiy] Ring modulators

[Richie Burnett]

When talking about rough-and-ready ring modulators it's perhaps worth 
mentioning the original ring modulator...
The diode-ring modulator from which the name comes:

http://en.wikipedia.org/wiki/Ring_modulator

As the text on Wikipedia explains, the diodes are alternately driven into 
conduction in pairs by the alternating carrier signal.  This has the effect 
of "patching" the input through to the output with periodically alternating 
polarity.

The text suggests that this is an on/off DPDT switching type of action that 
periodically reverses the polarity of the input signal being passed to the 
output.  Whilst the diode-ring modulator can be made to operate in 
"switching mode" when one of its ports is over driven, it is actually a 
proportional device.  A correctly scaled AC signal applied to the carrier 
port actually modulates the conductivity of the diodes in the ring causing a 
proportional modulation of the input signal's amplitude.

The more carrier voltage you apply to the diodes the more they become 
forward biased and their incremental resistance falls.  This passes more of 
the input signal through to the load at the output port.  If you reverse the 
carrier voltage the vertical pair of diodes now become gradually forward 
biased instead of the horizontal pair.  This passes progressively more and 
more of the input signal through to the output except now with reversed 
polarity.  You essentially have a rough-and-ready analogue multiplier.

It's amazing what you can achieve with a passive arrangement of 4 diodes and 
two transformers.  Most low-frequency ring-modulation applications are now 
either done with highly linear gilbert-cell based VCAs, or implemented 
digitally.  But for RF use particularly at higher frequencies miniature 
diode-ring mixers are alive and well, and at the heart of any 
super-hetrodyne radio receiver...

http://www.mini-circuits.com/pdfs/ADE-1.pdf

-Richie,